Total Petroleum Systems of the Illizi Province, 

Algeria and Libya—Tanezzuft-Illizi

By T.R. Klett

U.S. Geological Survey Bulletin 2202–A

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This report is only available online at:

http://greenwood.cr.usgs.gov/pub/bulletins/b2202–a/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

Published in the Central Region, Denver, Colorado

Manuscript approved for publication July 19, 2000

Graphics by the author

Photocomposition by Norma J. Maes

Edited by Lorna Carter
III

Contents

Foreword................................................................................................................................................ 1

Abstract ................................................................................................................................................. 2

Acknowledgments ............................................................................................................................... 2

Introduction........................................................................................................................................... 2

Province Geology ................................................................................................................................. 5

Tectonic History .......................................................................................................................... 5

Stratigraphy.................................................................................................................................. 8

Petroleum Occurrence .............................................................................................................. 11

Regional Exploration History .............................................................................................................. 11

The Tanezzuft-Illizi Total Petroleum System (205601) .................................................................... 11

 Source Rocks............................................................................................................................... 11

 Overburden Rocks....................................................................................................................... 13

Reservoir Rocks .......................................................................................................................... 13

Seal Rocks ................................................................................................................................... 13

 Trap Types in Oil and Gas Fields............................................................................................... 13

 Assessment of Undiscovered Petroleum by Assessment Unit ........................................... 13

Summary................................................................................................................................................ 14

References Cited.................................................................................................................................. 14

Appendix. Exploration-activity and discovery-history plots for the Tanezzuft-Illizi 

Structural/Stratigraphic Assessment Unit

Figures

1–3. Map showing approximate locations of:

 1. USGS-defined geologic provinces and major structures in north-

 central Africa ......................................................................................................................... 3

 2. Areal extent of total petroleum systems and Silurian source rocks

 (Tanezzuft Formation), and locations of stratigraphic cross

 sections, north-central Africa............................................................................................. 4

 3. Areal extent of assessment units within Illizi Province................................................. 6

 4. Stratigraphic cross sections through Illizi and neighboring

 provinces ..................................................................................................................................... 7

 5. Columnar section and stratigraphic nomenclature for Illizi, Triassic, and

 Ghadames (Berkine) Basins...................................................................................................... 9

 6. Tanezzuft-Illizi Total Petroleum System................................................................................... 12

 

Tables

[Tables and Appendix follow References Cited] 

 

1. Abbreviations, names, ages, and lithology of formations used in the total petroleum 

 systems events chart 

2. Reservoir properties of accumulations for the Tanezzuft-Illizi Structural/Stratigraphic

 Assessment Unit through 1995

3. Number and sizes of discovered fields for the Tanezzuft-Illizi Structural/Stratigraphic

 Assessment Unit through 1995

4. Estimated sizes, number, and coproduct ratios of undiscovered oil and gas fields for 

 the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit 

5. Estimated undiscovered conventional oil, gas, and natural gas liquids volumes for 

 oil and gas fields for the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit 
1

Foreword

This report was prepared as part of the U.S. Geological Survey World Petroleum Assessment 2000. The primary objective 

of World Petroleum Assessment 2000 is to assess the quantities 

of conventional oil, gas, and natural gas liquids outside the 

United States that have the potential to be added to reserves in 

the next 30 years. Parts of these assessed volumes reside in 

undiscovered fields whose sizes exceed the stated minimumfield-size cutoff value for the assessment unit, which is variable 

but must be at least 1 million barrels of oil equivalent. Another 

part of these assessed volumes occurs as reserve growth of fields 

already discovered. However, the contribution from reserve 

growth of discovered fields to resources is not covered for the 

areas treated in this report.

In order to organize, evaluate, and delineate areas to assess, 

the Assessment Methodology Team of World Petroleum Assessment 2000 developed a hierarchical scheme of geographic and 

geologic units. This scheme consists of regions, geologic provinces, total petroleum systems, and assessment units. For World 

Petroleum Assessment 2000, regions serve as organizational 

units and geologic provinces are used as prioritization tools. 

Assessment of undiscovered resources was done at the level of 

the total petroleum system or assessment unit.

The world was divided into 8 regions and 937 geologic 

provinces. These provinces have been ranked according to the 

discovered known oil and gas volumes (Klett and others, 1997). 

Then, 76 “priority” provinces (exclusive of the United States and 

chosen for their high ranking) and 26 “boutique” provinces 

(exclusive of the United States) were selected for appraisal of oil 

and gas resources. Boutique provinces were chosen for their 

anticipated petroleum richness or special regional economic or 

strategic importance.

A geologic province is an area having characteristic dimensions of hundreds of kilometers that encompasses a natural geologic entity (for example, a sedimentary basin, thrust belt, or 

accreted terrane) or some combination of contiguous geologic 

entities. Each geologic province is a spatial entity with common 

geologic attributes. Province boundaries were drawn as logically as possible along natural geologic boundaries, although in 

some places they were located arbitrarily (for example, along 

specific water-depth contours in the open oceans).

Total petroleum systems and assessment units were delineated for each geologic province considered for assessment. It 

is not necessary for the boundaries of total petroleum systems 

and assessment units to be entirely contained within a geologic 

province. Particular emphasis is placed on the similarities of 

petroleum fluids within total petroleum systems, unlike geologic provinces and plays in which similarities of rocks are 

emphasized.

The total petroleum system includes all genetically related 

petroleum that occurs in shows and accumulations (discovered 

and undiscovered) generated by a pod or by closely related pods 

of mature source rock. Total petroleum systems exist within a 

limited mappable geologic space, together with the essential 

mappable geologic elements (source, reservoir, seal, and overburden rocks). These essential geologic elements control the 

fundamental processes of generation, expulsion, migration, 

entrapment, and preservation of petroleum within the total petroleum system.

An assessment unit is a mappable part of a total petroleum 

system in which discovered and undiscovered oil and gas fields 

constitute a single relatively homogeneous population such that 

the methodology of resource assessment based on estimation of 

the number and sizes of undiscovered fields is applicable. A 

total petroleum system might equate to a single assessment unit. 

If necessary, a total petroleum system may be subdivided into 

two or more assessment units such that each assessment unit is 

sufficiently homogeneous in terms of geology, exploration considerations, and risk to assess individually. Differences in the 

distributions of accumulation density, trap styles, reservoirs, and 

exploration concepts within an assessment unit were recognized 

and not assumed to extend homogeneously across an entire 

assessment unit.

A numeric code identifies each region, province, total 

petroleum system, and assessment unit. The criteria for assigning codes are uniform throughout the project and throughout all 

publications of the project. The numeric codes used in this study 

are:

Unit Name Code

Region Middle East and North Africa 2

Province Illizi 2056

Total Petroleum System Tanezzuft-Illizi 205601

Assessment Unit Tanezzuft-Illizi Structural/ 20560101

 Stratigraphic 

A graphical depiction that places the elements of the total 

petroleum system into the context of geologic time is provided 

in the form of an events chart. Items on the events chart include 

(1) the major rock-unit names; (2) the temporal extent of sourcerock deposition, reservoir-rock deposition, seal-rock deposition, 

overburden-rock deposition, trap formation, generation-migration-accumulation of petroleum, and preservation of petroleum; 

and (3) the critical moment, which is defined as the time that 

best depicts the generation-migration-accumulation of hydrocarbons in a petroleum system (Magoon and Dow, 1994). The 

events chart serves only as a timeline and does not necessarily 

represent spatial relations. 

Probabilities of occurrence of adequate charge, rocks, and 

timing were assigned to each assessment unit. Additionally, an 

access probability was assigned for necessary petroleum-related 

Total Petroleum Systems of the Illizi Province, Algeria and 

Libya—Tanezzuft-Illizi

By T.R. Klett
2 Total Petroleum Systems of the Illizi Province, Algeria and Libya

activity within the assessment unit. All four probabilities, or 

risking elements, are similar in application and address the 

question of whether at least one undiscovered field of minimum 

size has the potential to be added to reserves in the next 30 years 

somewhere in the assessment unit. Each risking element thus 

applies to the entire assessment unit and does not equate to the 

percentage of the assessment unit that might be unfavorable in 

terms of charge, rocks, timing, or access.

Estimated total recoverable oil and gas volumes (cumulative production plus remaining reserves, called “known” volumes hereafter) quoted in this report are derived from 

Petroconsultants, Inc., 1996 Petroleum Exploration and Production database (Petroconsultants, 1996a). To address the fact that 

increases in reported known volumes through time are commonly observed, the U.S. Geological Survey (Schmoker and 

Crovelli, 1998) and the Minerals Management Service (Lore and 

others, 1996) created a set of analytical “growth” functions that 

are used to estimate future reserve growth (called “grown” volumes hereafter). The set of functions was originally created for 

geologic regions of the United States, but it is assumed that these 

regions can serve as analogs for the world. This study applied 

the Federal offshore Gulf of Mexico growth function (developed 

by the U.S. Minerals Management Service) to known oil and gas 

volumes, which in turn were plotted to aid in estimating undiscovered petroleum volumes. These estimates of undiscovered 

petroleum volumes therefore take into account reserve growth of 

fields yet to be discovered.

Estimates of the minimum, median, and maximum number, 

sizes, and coproduct ratios of undiscovered fields were made 

based on geologic knowledge of the assessment unit, exploration 

and discovery history, analogs, and, if available, prospect maps. 

Probabilistic distributions were applied to these estimates and 

combined by Monte Carlo simulation to calculate undiscovered 

resources.

Illustrations in this report that show boundaries of the total 

petroleum systems, assessment units, and extent of source rocks 

were compiled using geographic information system (GIS) software. The political boundaries shown are not politically definitive and are displayed for general reference only. Oil and gas 

field center points were provided by, and reproduced with permission from, Petroconsultants (1996a and 1996b).

Abstract

Undiscovered conventional oil and gas resources were 

assessed within a total petroleum system of the Illizi Province 

(2056) as part of the U.S. Geological Survey World Petroleum 

Assessment 2000. The Illizi Province is in eastern Algeria and a 

small portion of western Libya. The province and its total petroleum system coincide with the Illizi Basin. Although several 

total petroleum systems may exist within the Illizi Province, 

only one “composite” total petroleum system is identified. This 

total petroleum system comprises a single assessment unit.

The main source rocks are the Silurian Tanezzuft Formation 

(or lateral equivalents) and Middle to Upper Devonian mudstone. The total petroleum system was named after the oldest 

major source rock and the basin in which it resides.

The estimated means of the undiscovered conventional 

petroleum volumes in the Tanezzuft-Illizi Total Petroleum 

System are 2,814 million barrels of oil (MMBO), 27,785 billion 

cubic feet of gas (BCFG), and 873 million barrels of natural gas 

liquids (MMBNGL).

Acknowledgments

I thank Philip Farfan and Francois Gauthier of Anadarko 

Algeria Corporation, Rob Hoar and Randie Grantham of Oryx 

Energy Company, and David Boote and Marc Traut of Occidental Oil and Gas Corporation for their suggestions, which greatly 

improved the text. I also thank Katharine Varnes, Gordon Dolton, and Gregory Ulmishek for their editorial review.

Introduction

Undiscovered conventional oil and gas resources were 

assessed within a total petroleum system of the Illizi Province 

(2056) as part of the U.S. Geological Survey (USGS) World 

Petroleum Assessment 2000. This study documents the geology, 

undiscovered oil and gas volumes, exploration activity, and discovery history of the Illizi Province.

The Illizi Province is a geologic province delineated by the 

USGS; it is located in eastern Algeria and extreme western 

Libya (fig. 1). The province encompasses approximately 

200,000 km2 (square kilometers) and coincides with the Illizi 

Basin. Neighboring geologic provinces, delineated by the 

USGS, are the Trias/Ghadames Basin (2054), Hamra Basin 

(2047), Tihemboka Uplift (2057), Hoggar (7041), and Grand 

Erg/Ahnet Basin (2058).

More than one total petroleum system may exist within the 

Illizi Basin (Boote and others, 1998). Data available for this 

study are insufficient to adequately determine the relative contribution of each system to individual accumulations and therefore 

preclude further subdivision. Consequently only one “composite” total petroleum system is described in this report, which 

contains all total petroleum systems, called the Tanezzuft-Illizi 

Total Petroleum System (fig. 2). Tanezzuft refers to the Tanezzuft Formation (Silurian), which is the oldest major source rock 

in the system; in the total petroleum system name, “Tanezzuft” is 

then followed by the basin name in which the total petroleum 

system exists. Due to scarcity of data, province and total petroleum system boundaries can only be approximately delineated 

and therefore are subject to future modification.

One assessment unit was defined in the Tanezzuft-Illizi 

Total Petroleum System; the assessment unit coincides with the 

total petroleum system (fig. 3). The assessment unit is named 

after the total petroleum system with a suffix of “Structural/

Stratigraphic.” This suffix refers to the progression from a structural and combination trap exploration strategy to a stratigraphic 

(subtle) trap exploration strategy.

The Illizi Province contains more than 4,600 million barrels (MMB) of known (estimated total recoverable, that is, 

cumulative production plus remaining reserves) petroleum liquids (approximately 3,700 million barrels of oil, MMBO, and 

900 million barrels of natural gas liquids, MMBNGL) and 

approximately 45,000 billion cubic feet of known natural gas 

(109 CFG or BCFG) (Petroconsultants, 1996a). The Illizi 
Introduction 3

Approximate limit of salt

Saharan Flexure

Oued Mya

Basin

Tilrhemt Arch

Hassi Atshan Arch

Gargaf Arch

Triassic

Basin

Illizi

Basin Idjerane-M'Zab Structural Axis

Azzene High

Allal High

Ougarta Range

Antiatlas Range

Hoggar Massif

Ahnet

Basin

Timimoun

Basin

Bechar/

Abadla

Basins

Mouydir

Basin

Sbaa

Basin

ALGERIA

LIBYA

TUNISIA

MOROCCO

Grand Erg/Ahnet

Province 2058

Trias/Ghadames

Province 2054

Illizi

Province 2056

Pelagian

Province 2048

Pelagian

Basin

Country boundary

Province boundary

Structural axis (crest)

Djoua Saddle

Hamra

Basin

Oued Namous

Dome

Maharez

Dome

Edjeleh

Anticline Tihemboka Arch

Amguid-Hassi Touareg

Structural Axis

Atlas Mountains

Benoud Trough

Province of interest

Mediterranean Sea

Mediterranean

Sea

Melrhir

Basin

Ghadames

(Berkine)

Basin

Talemzane - Gefara Arch

Approximate

limit of salt

Ain Rich

High

Zousfana

Saddle

Beni Abbes

Saddle

Saharan flexure 0 500 KILOMETERS

5

o

W 0

o 5o

E 10oE 15oE

20oN

25oN

30oN

35oN

Mouydir

Structural

Terrace

Figure 1. North-central Africa, showing USGS-defined geologic provinces and major structures (modified from Aliev and others, 1971; Burollet 

and others, 1978; Montgomery, 1994; Petroconsultants, 1996b; Persits and others, 1997).
4 Total Petroleum Systems of the Illizi Province, Algeria and Libya

A

A' B'

B

Tanezzuft-Timimoun

TPS

Tanezzuft-Ahnet

TPS

Tanezzuft-Sbaa

TPS

Tanezzuft-Melrhir

TPS

Tanezzuft-Oued

Mya TPS

Tanezzuft-Mouydir

TPS

Tanezzuft-Ghadames

TPS

Tanezzuft-Illizi

TPS

Country boundary

Total petroleum system boundary

Silurian source rock boundary

Oil field

Gas field

ALGERIA

LIBYA

TUNISIA

MOROCCO

T-Shaped Anticlinorium

Figure 2. North-central Africa, showing the areal extent of total petroleum systems and Silurian source rocks (Tanezzuft Formation), and locations of 

stratigraphic cross sections (modified from Petroconsultants, 1996b; Persits and others, 1997; Boote and others, 1998).

Tanezzuft-Benoud

TPS

TanezzuftBechar/Abadla

TPS

0 500 KILOMETERS

5

o

W 0

o 5o

E 10oE 15oE

20oN

25oN

30o

N

35oN

Total petroleum systems of interest
Province Geology 5

Province contains the giant Tin Fouye-Tabankort and Zarzaitine oil fields.

Province Geology

The Tanezzuft-Illizi Province coincides with the Illizi Basin 

and is bounded on the north by the Ghadames (Berkine) Basin, 

on the east by the Tihemboka Arch, on the south by the Hoggar 

Massif, and on the west by the Amguid-Hassi Touareg structural 

axis.

The south-to-north plunging Tihemboka Arch separates the 

Illizi Basin from the Hamra Basin to the east (fig. 1) and is 

located near the Algerian-Libyan border. Along the western 

boundary, the Amguid-Hassi Touareg structural axis separates 

the Illizi Basin from the Mouydir Basin of the Grand Erg/Ahnet 

Province.

The boundary between the Illizi and Ghadames (Berkine) 

Basins is defined by a break or hinge line in the slope of the 

basement rocks. This hinge line was responsible for separating 

much of the petroleum generation, migration, and accumulation 

between the two basins (fig. 4A). The Illizi Basin is presently 

perched on a basement high and is not as deep as the neighboring Ghadames (Berkine) and Hamra Basins. The Illizi-Ghadames (Berkine) Basin boundary represents the approximate 

extent of Triassic and Jurassic evaporites that were deposited 

within the Mesozoic-aged Triassic Basin.

Tectonic History

The regional stratigraphy is continuous across North Africa, 

but petroleum generation, migration, and entrapment within 

each total petroleum system have been controlled by the tectonic 

history of individual basins. Deformational events in the region, 

most of them minor, are recorded by unconformities reflecting 

basin tilting, uplift, and erosion of intracratonic structural axes at 

various times throughout the Phanerozoic. The main deformational events occurred in the Precambrian to Early Cambrian 

(Pan African event), Late Silurian to Early Devonian, Late Devonian (Frasnian event), Carboniferous to Permian (Hercynian 

event), Early Jurassic, Early Cretaceous (Aptian, Austrian 

event), Late Cretaceous, and Tertiary (Eocene to Oligocene, 

Pyrenean event) (Aliev and others, 1971; Peterson, 1985; Boudjema, 1987; van de Weerd and Ware, 1994).

Throughout most of the Paleozoic, North Africa was a single depositional basin on the northern shelf of the African craton 

(Aliev and others, 1971; van de Weerd and Ware, 1994). The 

basin generally deepened northward where deposition and 

marine influence were greater (Daniels and Emme, 1995). Some 

gentle but large structures existed in this area throughout the 

Paleozoic and affected the thickness of the sedimentary cover 

(Aliev and others, 1971; van de Weerd and Ware, 1994). There 

was a general conformity of structure throughout most of the 

Paleozoic until the Hercynian event. In the Late Silurian and 

Early Devonian, Laurasia separated from Gondwana resulting in 

minor deformation, uplift, and local erosion (Aliev and others, 

1971; Boote and others, 1998). Many of the basins and uplifts 

preserved today were initially developed during this event from 

earlier structures (Peterson, 1985). Later, in the Middle to Late 

Devonian, the initial collision of Laurasia and Gondwana began 

resulting in erosion and further modification of preexisting structures (Boote and others, 1998).

Repeated minor uplift of the Tihemboka Arch and AmguidHassi Touareg structural axis occurred in the Late Silurian 

through the Devonian, with resulting local erosion and gradual 

development of the Illizi as a discrete basin (Aliev and others, 

1971; van de Weerd and Ware, 1994; Boote and others, 1998).

The Hercynian event marks the collision between Laurasia 

and Gondwana and caused regional uplift, folding, and erosion 

(Aliev and others, 1971; Boote and others, 1998). Paleozoic 

basins that were delineated by earlier tectonic events were modified, resulting in the development of several intracratonic sag 

and foreland basins (Aliev and others, 1971; van de Weerd and 

Ware, 1994; Boote and others, 1998). 

Several transgressive-regressive cycles occurrred throughout the Paleozoic. Two major flooding events, one in the Silurian and the other in the Late Devonian, were responsible for 

the deposition of source rocks (Aliev and others, 1971; Boudjema, 1987). Many of the prograding fluvial, estuarine, deltaic, 

and shallow marine sands that were deposited during these 

cycles are now reservoirs (Aliev and others, 1971).

Some petroleum may have been generated during the Carboniferous Period within the Paleozoic depocenter southwest of 

the Illizi Basin, but uplift caused petroleum generation to cease. 

Subsequent erosion probably removed or dispersed most petroleum that had accumulated (Tissot and others, 1973).

During the early Mesozoic, extensional movements caused 

by the opening of the Tethys and Atlantic oceans developed a 

cratonic sag basin called the Triassic Basin. The depocenter was 

superimposed on some of the Paleozoic basins (Aliev and others, 

1971; Boudjema, 1987). Triassic fluvial sands followed by a 

thick Triassic to Jurassic evaporite section were deposited within 

the sag basin (Aliev and others, 1971; Boudjema, 1987). Sandstones resulting from the fluvial deposition are major reservoirs, 

and the evaporites provide a regional seal for these fluvial and 

Paleozoic reservoirs (Aliev and others, 1971). Clastic then carbonate deposition occurred throughout the remainder of the 

Mesozoic over much of central North Africa (Aliev and others, 

1971; Boudjema, 1987). Sediment deposition gradually diminished in the Tertiary over most of the area (Aliev and others, 

1971; Peterson, 1985; Boudjema, 1987). Much of the petroleum 

present in Saharan Africa was generated in the Cretaceous and 

early Tertiary (Daniels and Emme, 1995; Makhous and others, 

1997).

 Transpressional movements (wrenching) during Austrian 

deformation reactivated older structures such as those along the 

Tihemboka Arch and Amguid-Hassi Touareg structural axis, 

causing local uplift and erosion (Claret and Tempere, 1967; 

Aliev and others, 1971). The southern portion of the Illizi Basin 

was unroofed at this time and was never again reburied 

(Macgregor, 1998; Boote and others, 1998; van de Weerd and 

Ware, 1994).

The initial stages of the Africa-Arabia and Eurasia collision 

during Late Cretaceous to middle Tertiary caused compressional 

movements and uplift (Peterson, 1985; Guiraud, 1998). 

These movements tilted the Triassic Basin to its present configuration (Aliev and others, 1971; Boote and others, 1998). 
6 Total Petroleum Systems of the Illizi Province, Algeria and Libya

LIBYA

Illizi Basin

Tanezzuft-Illizi 

Structural/Stratigraphic

150 KILOMETERS

ALGERIA

10o 8 E o

6 E o

E

28o

N

30o

N

26o

N

Country boundary

Total petroleum system boundary

Silurian source rock boundary

Oil field

Gas field

Geologic province boundary

Silurian absent

Silurian absent

Figure 3. Areal extent of assessment units within the Illizi Province (modified from Petroconsultants, 1996b; Persits and others, 1997).
Province Geology 7

0 100 KILOMETERS

NORTH SOUTH

GHADAMES (BERKINE)

BASIN

ILLIZI BASIN

0

1

2

3

4

5

6

7

KM

TERTIARY

UPPER CRETACEOUS

LOWER CRETACEOUS

JURASSIC

UPPER TRIASSIC

ORDOVICIAN

BASEMENT

CAMBRIAN

SILURIAN

DEVONIAN

CARBONIFEROUS

SILURIAN

DEVONIAN

A

SALIFEROUS UNITS

Tanezzuft-Ghadames

TPS (205403)

Tanezzuft-Illizi

TPS (205601) A'

TALEMZANE-GEFARA

ARCH

B-B'

Figure 4. Stratigraphic cross sections through Trias/Ghadames and Illizi Provinces. A, North-to-south stratigraphic cross section 

through the Ghadames (Berkine) and Illizi Basins (modified from van de Weerd and Ware, 1994, after Aliev and others, 1971).

Regional seal

Sections containing source rocks

Sections containing reservoir rocks

Fault

Unconformity

EXPLANATION

A

WEST EAST

0

1

2

3

4

5

KM

B

TERTIARY

UPPER CRETACEOUS

DEVONIAN

SILURIAN

CAMBRIAN-ORDOVICIAN

BASEMENT

SILURIAN

DEVONIAN

CARBONIFEROUS

BASEMENT

LOWER CRETACEOUS

AHNET

BASIN

IDJERANE-M'ZAB

STRUCTURAL AXIS

AMGUID-HASSI TOUAREG

STRUCTURAL AXIS

TIHEMBOKA

ARCH

ILLIZI

BASIN

MOUYDIR

BASIN

Tanezzuft-Mouydir B'

TPS (205804)

Tanezzuft-Illizi

TPS (205601)

A-A'

0 100 KILOMETERS

JURASSIC

CAMBRIANORDOVICIAN

CAMBRIANORDOVICIAN

Sections containing source rocks

Sections containing reservoir rocks

Fault

Unconformity

EXPLANATION

JURASSIC

Figure 4—Continued. Stratigraphic cross sections. B, West-to-east stratigraphic cross section through the Mouydir and Illizi Basins (modified 

from Makhous and others, 1997).

B
8 Total Petroleum Systems of the Illizi Province, Algeria and Libya

Petroleum was generated within the Illizi Basin from the 

Middle to Late Jurassic to early Tertiary (Daniels and Emme, 

1995). Some spillage or secondary migration occurred (Boote 

and others, 1998; van de Weerd and Ware, 1994). Many of the 

structural traps were formed by vertical movements of the basement during Mesozoic and Tertiary deformational events 

(Echikh, 1998).

Stratigraphy

The regional stratigraphy of lower Paleozoic sections is 

generally continuous, but the Devonian and overlying sections 

show more localized depositional systems. Stratigraphic 

nomenclature varies among the Saharan basins and countries. 

This study primarily uses nomenclature given in Boudjema 

(1987), Montgomery (1993), and Echikh (1998). Columnar sections, stratigraphic nomenclature, and correlations are shown in 

figure 5.

Principal source rocks are the Silurian Tanezzuft Formation 

and Middle to Upper Devonian mudstone (Givetian to Famennian) (Tissot and others, 1973; Daniels and Emme, 1995). Other 

minor or relatively unimportant source rocks are also present but 

contributed significantly less petroleum than did the Silurian or 

Middle to Upper Devonian mudstone (Daniels and Emme, 1995; 

Boote and others, 1998). Reservoir rocks include sandstone of 

Cambrian-Ordovician, Silurian, Devonian, and Carboniferous 

age. Intraformational Paleozoic marine mudstone provides the 

primary seals.

During the late Precambrian and Early Cambrian, erosion 

of a preexisting craton to the south occurred due to uplift during 

the Pan African deformational event. Eroded sediments were 

deposited northward as alluvial and fluvial deposits and make up 

the thick Cambrian sandstone of the Hassi Leila Formation. The 

Hassi Leila Formation and its lateral equivalents, the Hassaouna 

and Hassi Messaoud Formations, are major oil and gas reservoirs (Petroconsultants, 1996a; van de Weerd and Ware, 1994).

The Lower Ordovician In Kraf Formation unconformably 

overlies the Hassi Leila Formation. The In Kraf Formation contains mudstone and sandstone laterally equivalent to the Zone 

des Alterances, Argile d'El Gassi, and Gres d'El Atchane 

Formations of the Triassic Basin (fig. 5). The Argile d'El Gassi 

and equivalents may be locally minor source rocks (Makhous 

and others, 1997; Malla and others, 1997).

Overlying the In Kraf Formation is the quartz-rich, marine 

to marginal marine sandstone of the Hamra Formation (van de 

Weerd and Ware, 1994). Above the Hamra Formation is the 

Middle Ordovician Edjeleh Formation. The Edjeleh Formation 

is a marine sandstone that is a local facies of the Argiles d'Azzel 

mudstone (D. Boote, oral commun., 1999). Both the Hamra and 

Edjeleh Formations are important oil and gas reservoirs in the 

Illizi Province. Upper Ordovician to Lower Silurian marine and 

glacial mudstone and fine-grained sandstone of the Gara Louki 

Formation overlie the Edjeleh Formation.

Overlying the Gara Louki Formation is the organic-rich, 

graptolitic, marine mudstone of the Silurian Tanezzuft 

Formation. In the Illizi Basin, the Tanezzuft is sometimes 

referred to as “Argileux.” The Tanezzuft Formation, a principal 

source rock, was deposited during a major regional flooding 

event and contains mostly sapropelic and mixed (type I and II) 

kerogen (Daniels and Emme, 1995; Makhous and others, 1997). 

The present-day total organic carbon (TOC) content ranges from 

less than 2 to 9 percent across the Illizi Basin, but may be 

reduced by as much as one-half due to increased thermal maturity (Daniels and Emme, 1995). The TOC content is greatest at 

the base of the section (Daniels and Emme, 1995). The thickness, richness, and kerogen type of this source rock are regionally variable and dependent on paleogeography (Daniels and 

Emme, 1995). The thickness of the Tanezzuft Formation, before 

Hercynian erosion, ranged from about 200 m to greater than 500 

m (Boote and others, 1998).

The Tanezzuft Formation grades upward into sandstone and 

mudstone of the Silurian Zone de Passage. These sediments 

were deposited in marine to marginal marine environments and 

are laterally equivalent to the Acacus sandstone.

Devonian rocks unconformably overlie the Upper Silurian 

sediments. Devonian rocks consist of interbedded marine sandstone and mudstone. Sandstone members within the Devonian 

section are each given a code, F2 to F6 with F6 being the oldest 

and F2 the youngest (fig. 5). These sandstone members are principal oil and gas reservoirs in the Illizi Province.

Middle to Upper Devonian mudstone is another major 

source rock, particularly the Frasnian-aged mudstone, which is 

the richest in this interval (Daniels and Emme, 1995). This mudstone, like the Silurian Tanezzuft Formation, was deposited during a major regional flooding event and contains mostly 

sapropelic and mixed (type I and II) kerogen (Daniels and 

Emme, 1995; Makhous and others, 1997). Distribution of richness and kerogen type were presumably controlled by paleogeography. The present-day TOC content generally ranges from 

about 2 to 4 percent but can reach 7 percent (Daniels and Emme, 

1995). Thickness of Middle to Upper Devonian mudstone 

ranges from about 200 m to more than 400 m near the Hoggar 

Massif (Daniels and Emme, 1995).

The Lower to lower Middle Carboniferous rocks consist of 

cycles of limestone or mudstone, siltstone, sandstone, and conglomerate representing deltaic and shallow marine deposition. 

Upper Middle to Upper Carboniferous rocks consist of limestones, marls, dolostones, and gypsiferous mudstone that were 

deposited in evaporitic shallow-marine and tidal environments. 

As with Devonian rocks, Carboniferous formations are divided 

into members, each having a code from A to F, with A being the 

oldest and F the youngest (fig. 5).

No Permian rocks are present in the Illizi Province. Hercynian deformation started during the Late Carboniferous and 

continued into the Early Permian. Uplift caused deep erosion of 

Carboniferous through Devonian sections on the bounding structural highs and the basin began to assume its modern form.

The lower Mesozoic transgression, which swept across the 

eroded Hercynian peneplain from the northeast, did not reach 

the Illizi Province until very Late Triassic and Jurassic. The Triassic to Lower Cretaceous section is represented by nonmarine 

clastic rocks passing up into marginal marine clastics and then 

Figure 5 (next two pages). Columnar section and stratigraphic nomenclature for Illizi, Triassic, and Ghadames (Berkine) Basins (modified from 

Boudjema, 1987). Major reservoir rocks are shown in yellow, source 

rocks in gray, and seals in red.
Province Geology 9

InfraCambrian Cambrian Ordovician Silurian Devonian Carboniferous System

Stage

Stephanian

Westphalian

Namurian

Visean

Tournaisian

Famen. -Frasnian

Givetian -

Eifelian

Emsian

Siegenian -

Gedinnian

Cardocian

Llandeilian -

Llanvirnian

Arenigian

Tremadocian

Cambrian-Ordovician

Triassic Basin

(Boudjema,

1987)

Ghadames

(Berkine) and

Hamra Basins

(Montgomery,

1994; Echikh, 1998)

Tiguentourine

El Adeb Larache

Dembaba

Assed Jeffar Oubarakat

Assekaifaf

Issendjel

Mrar

Gara Mas Melouki Tahara (Shatti)

Tin Meras Aouinet Ouenine

Ouan Kasa Orsine

Hassi Tabankort Tadrart

Acacus

Oued Imirhou Tanezzuft

Gres de Remada

Argile Microcgl.

Memouniat

Melez Chograne

Gres d'Oued Saret

Argiles d'Azzel

Gres de Ouargla

Haouaz Quartzites De

Hamra

Gres d'El Atchane

Argile d'El Gassi

Zone des Alternances

Ri

Ro

R2

R3

Socle Infra Tassilian/

Mourizidie

Hassaouna

and

Mourizidie

Illizi Basin

(van de Weerd

and Ware,

1994)

"Argileux"

Gara Louki

Edjeleh

Hamra

In Kraf

Hassi Leila

General

lithology

(Boudjema,

1987)

Description

(Boudjema, 1987)

Hercynian Unconformity

Mudstone, limestone, and gypsum

Limestone, gypsum, and mudstone

Limestone and sandstone

Limestone and sandstone with concretions

Mudstone and sandstone

Limestone and mudstone

Sandstone

Mudstone Frasnian Unconformity

Sandstone

Mudstone and limestone

Mudstone and sandstone

Sandstone

Late Silurian-Early Devonian Unconformity

Sandstone and mudstone

Black mudstone with graptolites

Sandstone

Microconglomeratic mudstone Glacial Unconformity

Limestone, sandstone, and mudstone

Silty black mudstone

Sandstone

Sandstone

Sandstone and mudstone

Mudstone

Sandstone and mudstone

Sandstone

Sandstone and conglomerate

Pan-African Unconformity

Metamorphic and magmatic rocks

Hassi Messaoud

F6

F4-5

F3

F2

A

B

C

D

E

F

M'Kratta Complex

(Sbaa)

Zone de Passage

Strunian

Bir Tlacsin

Achebyat
10 Total Petroleum Systems of the Illizi Province, Algeria and Libya

System

Stage

Cretaceous

Lias

Dogger

Malm

Neocomian

Barremian

Aptian

Albian

Triassic Jurassic

Cenomanian

Turonian

Senonian

Paleog. Eocene Neogene

Miocene

Pliocene

General

lithology

(Boudjema,

1987)

Description

(Boudjema, 1987)

Conglomerate

Marl, gypsum, and sandstone

Gypsum, marl, limestone, sandstone,

and conglomerate

Pyrenean Unconformity Sandstone and marl

Limestone and marl

Anhydrite, limestone, dolostone, marl,

and mudstone

Limestone and dolostone

Anhydrite, dolostone, and marl

Austrian Unconformity

Sandstone, mudstone, and dolostone

Sandstone and mudstone

Sandstone and dolomitic mudstone

Anhydrite, limestone, dolostone, marl,

and sandstone

Mudstone, sandstone, limestone,

and anhydrite

Salt, anhydrite, and mudstone

Sandstone, mudstone, salt,

and volcanic rocks

Hercynian Unconformity

Triassic, Ghadames (Berkine),

and Hamra Basins

(Montgomery, 1994)

Al Hamra Group

Nefousa Group

Continental

Intercalaire

Scesciuch

(Shakshuk)

Tigi

Saliferous

Units

Argilo-Greseux

Superieur

(Gassi Touil)

Argilo-Carbonate

(Oulad Nsir)

Argilo-Greseux

Inferieur

(Nezla)

Cabao

Paleocene

Zarzaitine

Inferieur

Zarzaitine

Superieur

Zarzaitine

Moyen

Serie

de Taouratine

Serie

D'In Akamil

Argile Gypse

Calcaire

Calcaire

Marnes

Illizi Basin

(Chaouchi and

others, 1998)

Oligocene
Regional Exploration History 11

into marine carbonate rocks forming a north-facing sedimentary 

wedge. The Cenozoic section is represented by thin, discontinuous Miocene-Pliocene nonmarine rocks and Quaternary 

sediments.

Petroleum Occurrence

Most of the oil and gas fields found by the end of 1995 in 

the Illizi Province are located in the north-central part of the 

Illizi Basin and along the Tihemboka Arch, near the Triassic 

Basin where Mesozoic generation and migration occurred. 

Most of these accumulations are within low-relief, commonly 

faulted anticlines (Boote and others, 1998; Petroconsultants, 

1996a). The number of accumulations in the southern and western portions of the province had been destroyed by uplift, inactive generation, erosion of reservoir and seal rocks, and water 

flushing during and following Hercynian deformation (Boote 

and others, 1998; Macgregor, 1998). There is a lower potential 

for new discoveries in these areas compared to the northern and 

eastern portions of the Illizi Basin.

Regional Exploration History

Exploration activity in Algeria fluctuated through time due 

to its war for independence from 1954 to 1962, nationalization 

of the oil industry from 1963 to 1971, and political and economic problems into the 1980’s (Traut and others, 1998; Montgomery, 1994). Libya also experienced fluctuations and 

discontinuities. Between 1963 and late 1980’s, Algeria had legislation regarding concession contracts and royalties that discouraged exploration by foreign companies (Montgomery, 

1994). Since the late 1980’s, however, Algeria revised its legislation, encouraging foreign companies to explore and develop 

oil and gas resources (Davies and Bel Haiza, 1990; 

SONATRACH, c. 1992; Montgomery, 1994; Traut and others, 

1998).

Not all areas in Algeria and Libya were accessible for 

exploration. Shifting sand of Saharan African deserts presents 

technical difficulties in exploration and hazards in production 

operations (Echikh, 1998). Since the 1980’s, some of these technical difficulties in exploration have been resolved. Recent 

advances in gathering, processing, and reprocessing of seismic 

data allow exploration beneath sand-sea environments such as 

the Algerian Grand Erg Oriental where the Ghadames (Berkine) 

and Illizi Basins lie (van de Weerd and Ware, 1994; Macgregor, 

1998).

New discoveries can be easily brought on line without construction of major pipelines because a basic infrastructure has 

been established (SONATRACH, c. 1992). Algeria has an 

extensive pipeline network that connects most of the major producing areas to port cities in Algeria and Tunisia (Pennwell, 

1996). The pipelines allow transportation of oil, gas, and natural 

gas liquids. In Libya, an oil pipeline connects fields of the Ghadames (Berkine) and Hamra Basins to the port city of Az Zawiyah. An extension of this pipeline was completed in 1996 that 

connects producing areas within the Murzuk Basin with the 

coast (Traut and others, 1998; Pennwell, 1996), and another 

pipeline is planned to connect the Libyan portion of the Trias/

Ghadames Province to the coast (Arab Petroleum Research 

Center, 1996).

The Tanezzuft-Illizi Total Petroleum System 

(205601)

The Tanezzuft-Illizi Total Petroleum System is coincident 

with most of the Illizi Basin and Province. Paleozoic rocks, 

including the major source rocks, crop out along the Hoggar 

Massif (figs. 2 and 3). An events chart (fig. 6) summarizes the 

timing of sources, reservoirs, seals, trap development, and generation and migration of petroleum. Table 1 shows the formation 

names, ages, and lithology for abbreviations used in the events 

chart.

Source Rocks

The principal source rocks in the Tanezzuft-Illizi Total 

Petroleum System are the Silurian Tanezzuft Formation and 

Middle to Upper Devonian mudstone (Tissot and others, 1973; 

Daniels and Emme, 1995). In the Illizi Basin, the present-day 

TOC content of the Silurian Tanezzuft source rock typically 

ranges from 0.5 to 8 percent, but can be as much as 16 percent 

(Daniels and Emme, 1995; Makhous and others, 1997). Middle 

to Upper Devonian source rocks typically contain from 2 to 8 

percent TOC (Daniels and Emme, 1995; Makhous and others, 

1997). Equivalent vitrinite reflectance of Silurian source rocks 

ranges from 1.1 to 1.75 percent (Daniels and Emme, 1995) and 

the Middle to Upper Devonian, 1.1 to 1.3 percent Ro (Daniels 

and Emme, 1995). In the north-central, central, and southeastern 

portions of the total petroleum system, Silurian and Middle to 

Upper Devonian source rocks are presently in the peak to late oil 

generation phase (Daniels and Emme, 1995). In the northeastern 

portion, both source rocks are presently in the late oil to wet gas 

generation phase (Daniels and Emme, 1995). Silurian source 

rocks in the south and southwestern portions are presently in the 

wet to possibly the dry gas generation phase, whereas Devonian 

source rocks are presently in the late oil generation phase 

(Daniels and Emme, 1995).

Oil generation from both Silurian and Middle to Upper 

Devonian source rocks in the southern and western portion of 

the total petroleum system probably started and reached peak 

generative phases in the Carboniferous, but generation was 

halted during the Hercynian deformational event (Tissot and others, 1973; Daniels and Emme, 1995). Erosion resulting from 

Hercynian deformation probably removed most or all of the 

petroleum generated prior to the Hercynian event, and the source 

rocks in this portion of the total petroleum system were never 

again sufficiently buried to resume generation (Tissot and others, 

1973). Hercynian deformation was followed by Mesozoic subsidence and deposition to the north, which resulted in petroleum 

generation in the central, northern, and eastern portions of the 

total petroleum system (Tissot and others, 1973). Oil generation 
12 Total Petroleum Systems of the Illizi Province, Algeria and Libya ROCK UNIT SOURCE ROCK RESERVOIR ROCK SEAL ROCK OVERBURDEN ROCK TRAP FORMATION

GENERATIONPRESERVATION

CRITICAL MOMENT

PETROLEUM

SYSTEM EVENTS

GEOLOGICTIMESCALE CEN. MESOZOIC

TERT TR

E. L. E. E. L. L. E. E. E. M. M. M. L. L. L. L. E. E. M. L. L.

JUR CRET

ACCUMULATION MIGRATIONPALEOZOIC CAM ORD SIL DEV P PreC

0 100 200 300 400 500 600 150 250 350 450 550 50

TPS Name: Tanezzuft-Illizi (205601) Province Name: Illizi Basin (2056)

Author(s): Timothy R. Klett Date: 11-15-98

CARB

E O

M

HL

HM

ED

GL

TZ

F4-6

F3

F2

A, B

D

Glacial Unconformity

Pan-African Event

Pyrenean Event

Late Cretaceous Event

Austrian Event

Hercynian Event

Frasnian Event

Late SilurianEarly Devonian Event

TECTONIC EVENTS

AO

Figure 6. Events chart for Tanezzuft-Illizi Total Petroleum System. Names for 

abbreviations used in rock unit column are given in table 1. Gray boxes

indicate secondary or possible occurrences.
The Tanezzuft-Illizi Total Petroleum System (205601) 13

from Silurian source rocks in the central and southeastern portions of the total petroleum system peaked during the middle 

Cretaceous, whereas in the northern and eastern portions, generation probably peaked during the Middle to Late Jurassic 

(Daniels and Emme, 1995). Oil generation from Middle to 

Upper Devonian source rocks peaked during the Eocene in the 

central portion of the total petroleum system and Early to Late 

Cretaceous in the northern and eastern portions (Daniels and 

Emme, 1995). Petroleum most likely migrated laterally into 

adjacent or juxtaposed migration conduits and reservoirs. Some 

vertical migration occurred along faults or fractures in structurally deformed areas (Boote and others, 1998).

Overburden Rocks

Overburden rocks are variable across the area mainly due to 

nondeposition and erosion during the Hercynian, Austrian, and 

Pyrenean deformational events (fig. 4A and B). Carboniferous 

and Mesozoic rocks comprise most of the overburden for the 

Tanezzuft-Illizi Total Petroleum System. Large portions of Paleozoic section were removed by erosion during Hercynian deformation. Mesozoic rocks thin southward as a result of both 

original deposition and Tertiary uplift and unroofing in the south. 

As a result, Paleozoic rocks are thickest in the center of the basin 

(approximately 2,500 m, while the Mesozoic is thickest to the 

north and west (fig. 4A and B) (Bishop, 1975; Chiarelli, 1978; 

Boudjema, 1987; Montgomery, 1993; D. Boote, oral commun., 

1999). A thin Cenozoic section is present over the northern part 

of the total petroleum system.

Reservoir Rocks

The primary known reservoir rocks in the Tanezzuft-Illizi 

Total Petroleum System are Devonian sandstone. Devonian reservoirs include paralic to marine sandstone of the F2, F3, F4, F5, 

and F6 members (Petroconsultants, 1996a). Other known reservoir rocks include Cambrian-Ordovician, Silurian, and Carboniferous sandstone. Cambrian-Ordovician reservoirs include 

fluvial to marine sandstone of the Hassi Leila Formation, Hamra 

Formation, and marine and glacial sandstone of the Edjeleh Formation (Petroconsultants, 1996a, van de Weerd and Ware, 1994). 

Carboniferous reservoirs include the deltaic to marine sandstone 

of the B and D members, and Issendjel and Mrar Formations 

(Petroconsultants, 1996a). Only a few unnamed Silurian marine 

sandstone reservoirs are known (Petroconsultants, 1996a). 

Names of laterally equivalent rock units are shown in figure 5, 

and known reservoir properties are given in table 2.

Seal Rocks

Intraformational Paleozoic marine mudstone is the primary 

seal for reservoirs in the Tanezzuft-Illizi Total Petroleum System 

(Boote and others, 1998). Triassic to Jurassic evaporites, mudstone, and carbonate rocks that provide a regional top seal for 

reservoirs in the Ghadames (Berkine) Basin do not extend into 

the Illizi Province.

Trap Types in Oil and Gas Fields

Most of the accumulations discovered prior to 1996 are 

within anticlines and faulted anticlines (Echikh, 1998; Boote and 

others, 1998; Petroconsultants, 1996a, van de Weerd and Ware, 

1994). Accumulations within combination traps are present in 

the Illizi Basin. These are generally unconformity-related reservoir pinchouts across Hercynian or Austrian structures (Echikh, 

1998). Hydrodynamic traps may also be present but are not 

common (Echikh, 1998; Chiarelli, 1978; Aliev and others, 

1971). Accumulations within one of the largest oil fields of the 

Illizi Basin, Tin Fouye-Tabankort, are presumed hydrodynamically trapped (Echikh, 1998; Alem and others, 1998).

Assessment of Undiscovered Petroleum by 

Assessment Unit

One assessment unit was identified for the Tanezzuft-Illizi 

Total Petroleum System, called Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit (fig. 3). As of 1996, it contained 102 

fields. Of these discovered fields, 51 are oil fields, 38 are gas 

fields, and 13 fields are not classified because they contain less 

than 1 million barrels of oil equivalent (MMBOE) (based on 

USGS oil and gas field definitions). Combined, these fields contain 3,670 MMBO, 45,061 BCFG, and 898 MMBNGL, as 

known volumes (table 3) (Petroconsultants, 1996a). Minimum 

field sizes of 10 MMBO or 60 BCFG were chosen for this 

assessment unit based on the field-size distribution of discovered 

fields.

The exploration density as of 1996 was 16 new-field wildcat wells per 10,000 km2. The overall success rate as of 1996 

was 32 discoveries per 100 new-field wildcat wells (or about one 

discovery per three new-field wildcat wells). The success rate in 

terms of discoveries per number of new-field wildcat wells 

drilled remained relatively constant through time. Plots showing 

exploration activity and discovery history are presented in the 

appendix at the end of this report.

Exploration activity was not consistent through time: peaks 

occurred from the late 1950’s to mid-1960’s, in the early 1970’s, 

and in the mid-1980’s. The first major discoveries were the 

Edjeleh field in 1956 and the Zarzaitine field in 1957. Little 

exploration occurred and few discoveries were made between 

the early 1970’s and 1990. The sizes of discovered oil and gas 

fields have decreased somewhat through time and with respect 

to exploration activity. Some large accumulations are still being 

discovered, however. Exploration appears to be moderately 

mature across much of the area.

Until recently, only structural traps have been explored for 

oil and gas. Continued exploration of structural and combination traps is expected for the next 30 years. New exploration 

concepts could include the search for both structural and stratigraphic traps. Potential exists for the discovery of hydrodynamic traps along the flanks of structures that have already been 

explored (Chiarelli, 1978).

This study estimates that about one-third of the total 

number of fields (discovered and undiscovered) of at least the 

minimum size have been discovered. The estimated median 
14 Total Petroleum Systems of the Illizi Province, Algeria and Libya

size and number of undiscovered oil fields are 25 MMBO and 

67 fields; the same values for undiscovered gas fields are 200 

BCFG and 45 fields. The ranges of number, size, and coproduct-ratio estimates for undiscovered fields are given in table 4.

The estimated means of the undiscovered conventional 

petroleum volumes are 2,814 MMBO, 27,785 BCFG, and 873 

MMBNGL (table 5). In addition, the mean largest anticipated 

undiscovered oil and gas fields are 222 MMBO and 1,842 BCFG, 

respectively.

Summary

One total petroleum system and a corresponding assessment 

unit were identified, both coinciding with the Illizi Basin. This 

total petroleum system is called the Tanezzuft-Illizi.

The primary source rocks are the Silurian Tanezzuft Formation and Middle to Upper Devonian mudstone. In the southern 

and western portions, the source rocks reached peak generative 

phases during the Carboniferous. Elsewhere, peak oil generation 

occurred from the Middle to Late Jurassic to the Eocene. The 

primary reservoir rocks include Cambrian-Ordovician, Silurian, 

Devonian, and Carboniferous sandstone. Intraformational Paleozoic marine mudstone is the primary seal. Most of the known 

accumulations are within anticlines and faulted anticlines.

The estimated means of the undiscovered conventional 

petroleum volumes in the Tanezzuft-Illizi Total Petroleum System are 2,814 MMBO, 27,785 BCFG, and 873 MMBNGL. 

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[database available from Petroconsultants, Inc., P.O. Box 740619, 

Houston, TX 77274-0619].

Schmoker, J.W., and Crovelli, R.A., 1998, A simplified spreadsheet program for estimating future growth of oil and gas reserves: Nonrenewable Resources, v. 7, no. 2, p. 149–155.

SONATRACH, c. 1992, Exploration in Algeria: Algeria, Sur Presses Speciales U.A.F.A., 36 p.

Tissot, B., Espitalié, J., Deroo, G., Tempere, C., and Jonathan, D., 1973, 

Origin and migration of hydrocarbons in the eastern Sahara (Algeria): 6th International Meeting of Organic Geochemistry, reprinted in 

Demaison, G., and Murris, R.J., eds., Petroleum geochemistry and 

basin evaluation, American Association of Petroleum Geologists 

Memoir 25, p. 315–334.

Traut, M.W., Boote, D.R.D., and Clark-Lowes, D.D., 1998, Exploration history of the Palaeozoic petroleum systems of North Africa, in

Macgregor, D.S., Moody, R.T.J., and Clark-Lowes, D.D., eds., Petroleum geology of North Africa: Geological Society, London, Special 

Publication 132, p. 69–78.

van de Weerd, A.A., and Ware, P.L.G., 1994, A review of the East Algerian 

Sahara oil and gas province (Triassic, Ghadames and Illizi Basins): 

First Break, v. 12, no. 7, p. 363–373.
Table 1. Abbreviations, names, ages, and lithology of formations used in the total petroleum 

system events chart.

________________________________________________________________________

Code Formation name Age Lithology

________________________________________________________________________

A, B A and B Sandstone Lower Carboniferous Sandstone

Members

AO Aouinet Ouenine Middle to Upper Devonian Mudstone

D D Sandstone Member Lower Carboniferous Sandstone

ED Edjeleh Middle Ordovician Sandstone

F2 F2 Sandstone Member Upper Devonian Sandstone

F3 F3 Sandstone Member Middle Devonian Sandstone

F4-6 F4 to F6 Sandstone Lower Devonian Sandstone

Members

GL Gara Louki Upper Ordovician to Sandstone and

Lower Silurian mudstone

HM Hamra Lower Ordovician Sandstone

HL Hassi Leila Cambrian-Ordovician Sandstone

TZ Tanezzuft Silurian Mudstone

________________________________________________________________________
Table 2. Reservoir properties of discovered accumulations for the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit through 1995. [nd,

represents either no data or insufficient data. Data from Petroconsultants (1996a)]

USGS Code Depth Gross thickness Porosity Permeability

(meters) (meters) (percent) (millidarcies)

Minimum Mean Maximum Number Minimum Mean Maximum Number Minimum Mean Maximum Number Minimum Mean Maximum Number

205601 Tanezzuft-Illizi Total Petroleum System

20560101 Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit

Carboniferous 243 533 975 10 11 41 107 9 10 22 25 8 30 420 1,150 7

Devonian 152 1,522 2,955 41 4 43 396 91 11 21 30 38 2 892 7,000 24

Silurian 200 800 1,399 2 16 16 16 1 4 4 4 1 nd nd nd 0

Cambrian-Ordovician 453 1,817 3,100 120 7 55 176 28 4 9 15 103 10 552 800 102
USGS Code

Number of 

fields Known (discovered) volumes

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

205601 Tanezzuft-Illizi Total Petroleum System

20560101 Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit

Oil fields 51 3,406 14,795 263

Gas fields 38 262 30,244 635

Fields < 1 MMBOE 13 2 22 0

All fields 102 3,670 45,061 898

2056 Total

Oil fields 51 3,406 14,795 263

Gas fields 38 262 30,244 635

Fields < 1 MMBOE 13 2 22 0

All fields 102 3,670 45,061 898

Table 3. Number and sizes of discovered fields for the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit 

through 1995. [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, 

million barrels of NGL. Volumes reported are summed for oil and gas fields (USGS defined). Oil and gas fields 

containing known volumes below 1 million barrels of oil or 6 billion cubic feet of gas (BCFG) are grouped. Data 

from Petroconsultants (1996a)] 
USGS 

Code Size of accumulations Number of accumulations Gas-to-oil ratio NGL-to-gas ratio

(MMBO or BCFG) (CFG/BO) (BNGL/MMCFG)

Minimum Median Maximum Mean Shifted mean Minimum Median Maximum Mean Mode Minimum Median Maximum Mean Mode Minimum Median Maximum Mean Mode

205601 Tanezzuft-Illizi Total Petroleum System

20560101 Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit

Oil fields 10 25 500 28 38 10 67 200 75 14 2,350 4,700 7,050 4,705 4,700 11 22 33 22 22

Gas fields 60 200 4,800 263 323 8 45 84 45 44 20 40 60 40 40

Table 4. Estimated sizes, number, and coproduct ratios of undiscovered oil and gas fields for the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit. [MMBO, million barrels of oil; BCFG, billion 

cubic feet of gas; CFG/BO, cubic feet of gas per barrel oil, not calculated for gas fields; BNGL/MMCFG or BL/MMCFG, barrels of natural gas liquids or barrels of total liquids per million cubic feet of 

gas. BNGL/MMCFG was calculated for USGS-defined oil fields whereas BL/MMCFG was calculated for USGS-defined gas fields. Shifted mean, the mean size of the accumulation within a 

lognormal distribution of field sizes for which the origin is the selected minimum field size]
USGS 

Code MFS Prob. Undiscovered conventional volumes Largest anticipated undiscovered field

(0-1) Oil (MMBO) Gas (BCFG) NGL (MMBNGL) (MMBO or BCFG)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

205601 Tanezzuft-Illizi Total Petroleum System

20560101 Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit

Oil fields 10 1.00 601 2,520 6,000 2,814 2,655 11,444 29,734 13,235 55 244 685 291 86 205 419 222

Gas fields 60 1.00 5,809 14,241 24,286 14,550 213 551 1,055 582 753 1,633 3,696 1,842

All fields 601 2,520 6,000 2,814 8,464 25,685 54,020 27,785 269 795 1,740 873

2056 Total

Oil fields 601 2,520 6,000 2,814 2,655 11,444 29,734 13,235 55 244 685 291

Gas fields 5,809 14,241 24,286 14,550 213 551 1,055 582

All fields 601 2,520 6,000 2,814 8,464 25,685 54,020 27,785 269 795 1,740 873

Table 5. Estimated undiscovered conventional oil, gas, and natural gas liquids volumes for oil and gas fields for the Tanezzuft-Illizi Structural/Stratigraphic Assessment Unit. 

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, million barrels of natural gas liquids. Volumes of undiscovered NGL were 

calculated for oil fields whereas volumes of total liquids (oil plus NGL) were calculated for USGS-defined gas fields. Largest anticipated undiscovered field is in units of MMBO 

for oil fields and BCFG for gas fields. Results shown are estimates that are fully risked with respect to geology and accessibility. Undiscovered volumes in fields smaller than 

the selected minimum field size are excluded from the assessment. Means can be summed, but fractiles (F95, F50, and F5) can be summed only if a correlation coefficient of 

+1.0 is assumed]
APPENDIX

Exploration-activity and discovery-history plots for each of the assessment units. Two sets of 

plots and statistics are provided, one set showing known field sizes (cumulative production plus 

remaining reserves) and another showing field sizes upon which a reserve-growth function was 

applied (labeled grown). Within each set of plots, oil fields and gas fields are treated separately. 

The plots include:

• Cumulative Number of New-Field Wildcat Wells vs. Drilling-Completion Year

• Number of New-Field Wildcat Wells vs. Drilling-Completion Year

• Oil- or Gas-Field Size (MMBO or BCFG) vs. Oil- or Gas-Field Rank by Size (With 

Respect to Discovery Halves or Thirds)

• Number of Oil or Gas Fields vs. Oil- or Gas-Field Size Classes (MMBO or BCFG) (With 

Respect to Discovery Halves or Thirds)

• Volume of Oil or Gas (MMBO or BCFG) vs. Oil- or Gas-Field Size Classes

(MMBO or BCFG)

• Oil- or Gas-Field Size (MMBO or BCFG) vs. Field-Discovery Year

• Oil- or Gas-Field Size (MMBO or BCFG) vs. Cumulative Number of New-Field 

Wildcat Wells

• Cumulative Oil or Gas Volume (MMBO or BCFG) vs. Field-Discovery Year

• Cumulative Oil or Gas Volume (MMBO or BCFG) vs. Cumulative Number of NewField Wildcat Wells

• Cumulative Number of Oil or Gas Fields vs. Field-Discovery Year
• Cumulative Number of Oil or Gas Fields vs. Cumulative Number of New-Field 

Wildcat Wells

• Reservoir Depth, Oil or Gas Fields (m) vs. Field-Discovery Year

• Reservoir Depth, Oil or Gas Fields (m) vs. Cumulative Number of New-Field 

Wildcat Wells

• Gas/Oil, Oil Fields (CFG/BO) vs. Mean Reservoir Depth (m)

• NGL/Gas, Oil Fields (BNGL/MMCFG) vs. Mean Reservoir Depth (m)

• Liquids/Gas, Gas Fields (BL/MMCFG) vs. Mean Reservoir Depth (m)

• Number of Reservoirs in Oil Fields vs. API Gravity (Degrees)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

50

100

150

200

250

300

350

400

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

DRILLING-COMPLETION YEAR

CUM. NEW-FIELD WILDCAT WELLS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5

10

15

20

25

30

35

40

45

50

19561958196019621964196619681970197219741976197819801982198419861988199019921994

DRILLING-COMPLETION YEAR

NEW-FIELD WILDCAT WELLS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

0 2 4 6 8 10 12 14 16 18

OIL-FIELD RANK BY SIZE

KNOWN OIL-FIELD SIZE (MMBO)

First third of fields discovered

Second third of fields discovered

Third third of fields discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2

4

6

8

10

12

1 to <22 to <44 to <8

8 to <16

16 to <3232 to <64

64 to <128

128 to <256256 to <512

512 to <1,024

1,024 to <2,0482,048 to <4,0964,096 to <8,192

8,192 to <16,384

16,384 to <32,76832,768 to <65,536

65,536 to <131,072

131,072 to <262,144

>=262,144

KNOWN OIL-FIELD SIZE (MMBO)

OIL FIELDS (No.)

Third third of

fields

discovered

Second third

of fields

discovered

First third of

fields

discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

100

200

300

400

500

600

700

800

900

1,000

1 to <22 to <44 to <8

8 to <16

16 to <3232 to <64

64 to <128

128 to <256256 to <512

512 to <1,024

1,024 to <2,0482,048 to <4,0964,096 to <8,192

8,192 to <16,384

16,384 to <32,76832,768 to <65,536

65,536 to <131,072

131,072 to <262,144

>=262,144

KNOWN OIL-FIELD SIZE (MMBO)

VOLUME OF OIL (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

KNOWN OIL-FIELD SIZE (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

KNOWN OIL-FIELD SIZE (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. KNOWN OIL VOLUME (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. KNOWN OIL VOLUME (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

RESERVOIR DEPTH, OIL FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

RESERVOIR DEPTH, OIL FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

GAS/OIL, OIL FIELDS (CFG/BO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

200

400

600

800

1,000

1,200

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

NGL/GAS, OIL FIELDS (BNGL/MMCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

0 to <10 10 to

<20

20 to

<30

30 to

<40

40 to

<50

50 to

<60

60 to

<70

70 to

<80

80 to

<90

90 to

<100

>=100

API GRAVITY (DEGREES)

RESERVOIRS IN OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

0 2 4 6 8 10 12 14

GAS-FIELD RANK BY SIZE

KNOWN GAS-FIELD SIZE (BCFG)

First third of fields discovered

Second third of fields discovered

Third third of fields discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2

4

6

8

10

12

14

6 to <12

12 to <2424 to <4848 to <96

96 to <192

192 to <384384 to <768

768 to <1,536

1,536 to <3,0723,072 to <6,144

6,144 to <12,288

12,288 to <24,57624,576 to <49,15249,152 to <98,304

98,304 to <196,608

196,608 to <393,216393,216 to <786,432

786,432 to <1,572,864

>=1,572,864

KNOWN GAS-FIELD SIZE (BCFG)

GAS FIELDS (No.)

Third third of

fields

discovered

Second third

of fields

discovered

First third of

fields

discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

6 to <12

12 to <2424 to <4848 to <96

96 to <192

192 to <384384 to <768

768 to <1,536

1,536 to <3,0723,072 to <6,144

6,144 to <12,288

12,288 to <24,57624,576 to <49,15249,152 to <98,304

98,304 to <196,608

196,608 to <393,216393,216 to <786,432

786,432 to <1,572,864

>=1,572,864

KNOWN GAS-FIELD SIZE (BCFG)

VOLUME OF GAS (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

KNOWN GAS-FIELD SIZE (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

KNOWN GAS-FIELD SIZE (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. KNOWN GAS VOLUME (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. KNOWN GAS VOLUME (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5

10

15

20

25

30

35

40

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. GAS FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. GAS FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

RESERVOIR DEPTH, GAS FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

RESERVOIR DEPTH, GAS FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

70

80

90

100

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

LIQUIDS/GAS, GAS FIELDS (BL/MMCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

0 2 4 6 8 10 12 14 16 18

OIL-FIELD RANK BY SIZE

GROWN OIL-FIELD SIZE (MMBO)

First third of fields discovered

Second third of fields discovered

Third third of fields discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2

4

6

8

10

12

1 to <22 to <44 to <8

8 to <16

16 to <3232 to <64

64 to <128

128 to <256256 to <512

512 to <1,024

1,024 to <2,0482,048 to <4,0964,096 to <8,192

8,192 to <16,384

16,384 to <32,76832,768 to <65,536

65,536 to <131,072

131,072 to <262,144

>=262,144

GROWN OIL-FIELD SIZE (MMBO)

OIL FIELDS (No.)

Third third of

fields

discovered

Second third

of fields

discovered

First third of

fields

discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

200

400

600

800

1,000

1,200

1,400

1,600

1 to <22 to <44 to <8

8 to <16

16 to <3232 to <64

64 to <128

128 to <256256 to <512

512 to <1,024

1,024 to <2,0482,048 to <4,0964,096 to <8,192

8,192 to <16,384

16,384 to <32,76832,768 to <65,536

65,536 to <131,072

131,072 to <262,144

>=262,144

GROWN OIL-FIELD SIZE (MMBO)

VOLUME OF OIL (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

GROWN OIL-FIELD SIZE (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

GROWN OIL-FIELD SIZE (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. GROWN OIL VOLUME (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. GROWN OIL VOLUME (MMBO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

RESERVOIR DEPTH, OIL FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

RESERVOIR DEPTH, OIL FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

GAS/OIL, OIL FIELDS (CFG/BO)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

200

400

600

800

1,000

1,200

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

NGL/GAS, OIL FIELDS (BNGL/MMCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

0 to <10 10 to

<20

20 to

<30

30 to

<40

40 to

<50

50 to

<60

60 to

<70

70 to

<80

80 to

<90

90 to

<100

>=100

API GRAVITY (DEGREES)

RESERVOIRS IN OIL FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

0 2 4 6 8 10 12 14

GAS-FIELD RANK BY SIZE

GROWN GAS-FIELD SIZE (BCFG)

First third of fields discovered

Second third of fields discovered

Third third of fields discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

2

4

6

8

10

12

14

6 to <12

12 to <2424 to <4848 to <96

96 to <192

192 to <384384 to <768

768 to <1,536

1,536 to <3,0723,072 to <6,144

6,144 to <12,288

12,288 to <24,57624,576 to <49,15249,152 to <98,304

98,304 to <196,608

196,608 to <393,216393,216 to <786,432

786,432 to <1,572,864

>=1,572,864

GROWN GAS-FIELD SIZE (BCFG)

GAS FIELDS (No.)

Third third of

fields

discovered

Second third

of fields

discovered

First third of

fields

discovered
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5,000

10,000

15,000

20,000

25,000

6 to <12

12 to <2424 to <4848 to <96

96 to <192

192 to <384384 to <768

768 to <1,536

1,536 to <3,0723,072 to <6,144

6,144 to <12,288

12,288 to <24,57624,576 to <49,15249,152 to <98,304

98,304 to <196,608

196,608 to <393,216393,216 to <786,432

786,432 to <1,572,864

>=1,572,864

GROWN GAS-FIELD SIZE (BCFG)

VOLUME OF GAS (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

GROWN GAS-FIELD SIZE (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

1

10

100

1,000

10,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

GROWN GAS-FIELD SIZE (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. GROWN GAS VOLUME (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. GROWN GAS VOLUME (BCFG)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5

10

15

20

25

30

35

40

45

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

CUM. GAS FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

CUM. GAS FIELDS (No.)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

FIELD-DISCOVERY YEAR

RESERVOIR DEPTH, GAS FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 50 100 150 200 250 300 350 400

CUM. NEW-FIELD WILDCAT WELLS (No.)

RESERVOIR DEPTH, GAS FIELDS (m)
Tanezzuft-Illizi Structural/Stratigraphic, Assessment Unit 

20560101

0

10

20

30

40

50

60

70

80

90

100

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

MEAN RESERVOIR DEPTH (m)

LIQUIDS/GAS, GAS FIELDS (BL/MMCFG)